EDWIN E. WESTERFIELD, DENNIS 1. DUVEN, and LARRY L. WARNKE
DEVELOPMENT OF A GLOBAL POSITIONING
SYSTEM/SONOBUOY SYSTEM FOR DETERMINING
BALLISTIC MISSILE IMPACT POINTS
Testing of ballistic missile systems requires a method for accurately determining the position of
at-sea impact points. Historically, this information has been determined by analyzing the acoustic
signals received by a number of sonobuoys floating in the target area. The positions of the buoys
were determined by using acoustic ping signals emanating from Deep Ocean Transponders located
on the ocean bottom. An improved system is being developed by APL to locate the positions of the
buoys using signals received from the Global Positioning System. This will eliminate the need for
Deep Ocean Transponders along with the associated ship costs for installing, surveying, and maintaining them.
BACKGROUND
The Sonobuoy Missile Impact Location System has
been used for a number of years to determine where
reentry bodies, released by ballistic missiles, impact in
ocean target areas. An Air Force program was initiated over two years ago to develop an improved system
to provide the required information at lower cost. The
second phase of the three-phase development program
is nearing completion.
Figure 1 is an overview of the original Deep Ocean
Transponder I Sonobuoy Missile Impact Location System (DOT I SMILS), which makes use of several types
of sonobuoys that are dropped by mission support aircraft. A typical sonobuoy is 4.5 inches in diameter and
less than 36 inches long. As the buoy falls free from
the aircraft, a small drag parachute deploys and stabilizes the descent of the buoy into the water. At the time
of impact, the parachute is released and an antenna
is erected. In some buoys, the antenna is located in
a small balloon (float) assembly that is inflated by gas
from a pressure bottle in the sonobuoy. The balloon
provides extra flotation for the buoy and protects the
antenna from salt spray.
Concurrent with balloon inflation, the buoy releases
a hydrophone assembly that descends to a depth of
about 30 feet. The hydrophone picks up acoustic signals generated by other buoys and the sound of each
reentry vehicle impact and transmits that information
by means of a VHF radio link to the mission support
aircraft circling overhead. Some buoys within the array deploy a second hydrophone that injects an acoustic transponder command signal into the water.
Various types of buoys are used in the missile impact location system shown in Fig. 1. The velocimeter
buoy measures the velocity of sound in water, while
the bathythermograph buoy measures the temperature
Volume 5, Number 4, 1984
.,
Figure 1 - Overview of the DOT/SMILS system. Up to 12
sonobuoys are dropped in the target area by a range support
aircraft. Each sonobuoy is equipped with an acoustic trans·
mitter and a receiver. The acoustic information picked up
from the water is transmitted to the aircraft via a VHF radio
link. DOTs, previously implanted on the ocean bottom at a
known location by a ship, are used to determine the position
of the buoys by means of acoustic transmissions. The posi·
tion of the reentry body impact relative to the buoys is also
determined acoustically. When all the information is com·
bined, the geodetic position of the impact can be determined.
of the water as a function of depth. The interrogator
buoys are equipped with sonic transmitters that trans335
E. E. Westerfield et al. -
Improved Sonobuoy System for Missile Impact Location
mit acoustic command signals to DOTs located at
known positions on the ocean floor. Ten transponders
typically are used in a target area to provide the necessary coverage. The DOTs respond to interrogations .by
generating an acoustic signal, each DOT responding
on a different acoustic frequency . The pinger buoys
on the surface are key system components, both receiving and generating acoustic signals. They receive
acoustic transmissions from all buoys equipped with
acoustic transmitters and also detect the loud acoustic signals generated by the reentry bodies as they strike
the water.
Electronic equipment aboard the aircraft is used to
monitor the RF signals transmitted by all of the buoys
in the water. After being processed, these transmissions allow us to determine the time of arrival of each
acoustic signal received by each buoy, as well as the
time of acoustic pulse transmission by each buoy. This
information can be processed mathematically to yield
the position of each buoy relative to the others. In addition, whenever an interrogator buoy generates a
command transmission, all transponders within range
respond with a high-frequency reply. The responses
as received at the interrogator buoy enable an accurate
estimation to be made of the slant range from the buoy
to each DOT. When the transponders were originally
implanted on the ocean floor, their geodetic positions
were determined accurately, typically by means of the
Navy Navigation Satellite System. By combining that
position information with the slant range measurements, the geodetic position of each interrogator buoy
can be determined accurately. When the positions of
two or more interrogator buoys have been derived, the
geodetic positions of all the buoys in the system can
be determined because their relative positions are already known.
When a reentry body impact occurs, the resulting
acoustic signal is picked up by all buoys. If the relative times of arrival of the signals at the buoys are compared, the impact position can be determined to a high
degree of precision.
The DOT I SMILS system described above requires
that relatively large ships be used to plant the DOTs,
survey their positions, and perform essential maintenance in the target areas far from land . Currently,
the transponder position survey requires nearly a week,
principally because the Navy Navigation Satellite System is used as the primary position source. To obtain
the required accuracy, data from a minimum of 20 satellite passes should be taken and averaged; this requires
a number of days because of the system's orbital configuration. In the future, by using the Global Positioning System (GPS) (see the boxed insert) to perform the
survey function, the on-station time should be reduced
significantly. Relatively large ships will continue to be
required for survey and implant, however, because of
the remote locations. It is possible to replace inactive
DOTs in the array by means of aircraft. Such a system is the Air Launch Deep Ocean Transponder system under development for the Navy. It will reduce
maintenance costs for the DOT I SMILS array.
336
DEVELOPMENT OF THE
GPS/SMILS SYSTEM
Several years ago, the Air Force Western Space and
Missile Center at Vandenberg AFB investigated alternative approaches to the current system with the aim
of reducing or eliminating the cost of implanting the
DOTs. A concept called the Global Positioning System/Sonobuoy Missile Impact Location System (GPSI
SMILS) (Fig. 2) was found to be very promising, and
the Air Force selected APL to develop it.
In that system, the interrogator buoys and the DOTs
are replaced by buoys known as GPS transdigitizer
buoys (Fig. 3). The buoys not only perform the functions of the pinger buoys, but also receive signals from
the GPS satellites and amplify, filter, and downconvert the signals to a low frequency of 20 kilohertz.
The signals are then fed via a voltage comparator into
Pinger buoys (6)
G PS transd igitizer
buoys (3)
Figure 2 - Overview of the GPS/SMILS system. Buoys are
dropped in the target area by a range support aircraft. Three
buoys are equipped with transdigitizer assemblies that reo
ceive transmissions from GPS satellites, convert the signals
to digital format , and retransmit them to the support aircraft.
Hardware in the aircraft processes the signals and determines
the geodetic position of the three buoys. The other buoys
(known as pinger buoys) , as well as the GPS transdigitizer
buoys, are equipped to transmit and receive acoustic signals.
The received acoustic Signals are retransmitted to the aircraft for processing. The pOSition of the pinger buoys relative to the GPS transdigitizer buoys is determined by
acoustics. When a reentry body impact occurs, its position
relative to the buoys is also determined by acoustics. When
all the information is combined , the precise position of the
impact can be determined .
fohns Hopkins APL Technical Digest
E. E. Westerfield et al. -
Improved Sonobuoy System for Missile Impact Location
DESCRIPTION OF THE GLOBAL
POSITIONING SATELLITE SYSTEM
Float bag
Antennas
57 in .
Compressed air bottles
Transdigitizer electronics
The Navstar Global Positioning System is a spacebased radio positioning navigation and time transfer
system. Its development is being managed by a Joint
Program Office at the Space Division of the Air Force
Systems Command in Los Angeles. The management
team is comprised of Department of Defense, Department of Transportation, and NATO personnel. When
fully implemented, the system will consist of a minimum of 18 satellites orbiting at an altitude of 10,900
miles. At that height, the satellites have a period of
12 hours. The satellites will be arranged in six orbital
rings of three satellites each. The rings are at an inclination of 55 relative to the earth's equator.
All the satellites continuously transmit at frequencies of 1575.42 megahertz (MHz) (Ll) and 1227.6 MHz
(L2). The 1575.42 MHz frequency is modulated in
quadrature by two pseudorandom-noise coded signals,
while the 1227.6 MHz signal is modulated by a single
pseudorandom-noise code. To allow the user to determine the position of the spacecraft as a function
of time, each satellite transmits a message data stream
that contains the orbital elements plus data to allow
compensation for spacecraft clock errors. The message data are added to both of the pseudorandom
codes.
0
Hydrophones
Weight
Figure 3 - The GPS/SMILS transdigitizer buoy. The trans·
digitizer buoy is supported in the water by a float assembly
that also contains the GPS antenna and the antenna used
to receive commands and transmit data to the aircraft. The
middle portion of the assembly houses the buoy electron·
ics, including the transdigitizer, command receiver, data
transmitter, acoustic transmitter, and processor. The batter·
ies are located below this assembly. Receiving and transmit·
ting hydrophones are deployed below the buoy.
a I-bit latch circuit to generate a digital data stream.
These data, along with digitized acoustic data, are used
to modulate a carrier for transmission of the signals
to the system aircraft over a VHF link.
Equipment in the aircraft receives the signals from
the trans digitizer buoys, separates out the acoustic data
from the GPS data, and processes all the data. The
GPS data are fed into a number of tracking channels,
each of which tracks the signal from one satellite. The
output of the channels is pseudorange and pseudorange-rate for the satellite-buoy-aircraft path and GPS
message data that describe the satellite orbits. The data
can be processed mathematically to provide accurate
position data for the trans digitizer buoys. The positions of the pinger buoys in relation to the transdigitizer buoys are then obtained using acoustics, in a fashion
similar to that used for the original system. The
GPS/ SMILS system will function more automatically than the earlier system. The advantages are that no
ship is required and there is target flexibility, thereby
reducing costs.
DESCRIPTION OF THE
TRANSDIGITIZER BUOY
Figure 3 is an exploded view of the transdigitizer
buoy following inflation of the float bag and release
Volume 5, Number 4, 1984
of the hydrophones (acoustic transducers). The antenna system, located in the float bag, must provide for
receipt of signals from the GPS satellites, receipt of
VHF command signals from the aircraft, and transmission of the data back to the aircraft at VHF. The
antenna system consists of a backfire helix antenna for
reception of the 1575.42 megahertz (MHz) transmission from the satellites. A monopole antenna located
at the center of the helix is used both for reception of
VHF command signals and transmission of data to the
aircraft. The tops of both the helix and the monopole
antennas are tied to the top of the float assembly. At
the time the buoy is dropped from the aircraft, the
float bag and antenna are folded into a compartment
at the top of the buoy. When the buoy strikes the water, the float is inflated by the release of air from bottles of compressed air located underneath the float
storage area, and the antennas are erected automatically.
Underneath the float storage area is a group of
boards containing the electronics for the transdigitizer, including the transmitter and the command receiver. There is also a board containing circuitry for
processing the acoustic data received from the hydrophone. The board at the bottom of the stack houses
the acoustic transmitter. Below the electronics is a
compartment housing the batteries.
At the bottom of the buoy is a chamber that houses
the acoustic transducer, a damper, and the cable prior to deployment. One transducer serves as a damper. Part of the cable connecting the buoy to the damper
337
E. E. Westerfield et al. - Improved Sonobuoy System for Missile Impact Location
is elastic and can readily be stretched. The function
of the damper and the cable is to keep the hydrophones
stationary while the buoy rides up and down on the
ocean swells, in order to reduce the water noise that
would occur if the hydrophones were riding up and
down in the water following the vertical motion of the
buoy.
Figure 4 is a block diagram of the electronics within the trans digitizer. The 1575.42 MHz signals received
by the antenna are filtered, amplified, and heterodyned
to a first IF frequency of 75.42 MHz. This signal is
further amplified and heterodyned down to a frequency of 20 kilohertz. It is then passed through a 1 MHz
low-pass filter / amplifier and thence to a zero crossing detector that provides a digital output. This signal is sampled by means of a flip -flop that is clocked
by a 2 MHz signal. The digital signal is then fed into
one input of a quadraphase modulator where it modulates one phase of the uplink carrier.
The second input to the quadraphase modulator is
an asynchronous digital data stream generated by processing the acoustic data received by the hydrophone.
The signal from the modulator is amplified to provide
an RF output power of 2 watts to the antenna.
AIRCRAFT EQUIPMENT
Figure 5 is a block diagram of the equipment required in the aircraft to support the operation of the
system. The equipment will be housed in a modified
Boeing 707 aircraft along with other range equipment.
The aircraft will be equipped with the electronics
shown in the diagram and also with the facilities to
carry and launch sonobuoys. It will also have special
antennas to receive the signals from the buoys and an
antenna to receive signals directly from the GPS satellites. The former antennas will be mounted under the
fuselage, while the latter will be top mounted.
The signals received from the buoys are fed to two
receiver groups. The first is a standard antisubmarine
warfare receiver used for receiving the signals from all
buoys except the transdigitizers. The transmissions
from the transdigitizer buoys are processed by a specially designed 6-channel uplink receiver that filters
and amplifies the signals and separates the acoustic
data from the GPS data. The GPS and acoustic data
are recorded by a standard 14-track analog recorder.
The GPS data are also fed to a 12-channel digital
tracking system. Twelve channels are required in order to allow the simultaneous processing of four satellite signals received from three trans digitizer buoys.
The system also computes the position of each transdigitizer buoy.
A second bank of digital trackers is used to process
the signals received directly from the satellites. The
principal function of this subsystem is to determine
the effect that ionospheric refraction has on the
VHF
antenna
Low pass
filter/
amp lifier
Zero
75.4 MHz
2 MHz
u
"Cl.l
. - +-'
+-' +-'
~·E
o
(J')
u c
«~
Computercompatible
recorder
Transmitting
hydrophone
Figure 4 - Block diagram of the buoy electronics. The signal received from the GPS satellite at 1575.42 MHz is filtered ,
amplified , and converted to low frequency by mixing with signals generated in the frequency synthesizer. The signal is
then sampled and converted to two-level digital format. The
acoustic data picked up from the water are amplified, filtered ,
and converted to digital format. These and the GPS data are
transmitted to the aircraft via an RF link. The buoy is also
equipped with a command receiver and an acoustic transmitter.
338
Figure 5 - Block diagram of the aircraft SMILS system.
Equipment aboard the aircraft controls the buoys in the water by means of a command transmitter, and receives both
acoustic and digitized GPS satellite data from the buoys. The
relayed GPS signals are tracked in the GPS tracker and the
positions of the three buoys equipped with transdigitizers
are determined. The acoustic data are processed in the acoustic processor to provide the relative position of each buoy
to all the other buoys. The system is equipped with necessary recorders and operator displays.
Johns Hopkins APL Technical Digest
E. E. Westerfield et af. -
1575.42 MHz transmission from each satellite. -This
is accomplished by processing the 1575.42 and 1227.6
MHz pseudorange and range-rate signals from each
spacecraft to estimate the refraction effect. The refraction data are used to correct the 1575.42 MHz signals
received at the trans digitizer buoys. Studies have indicated that at the maximum range of the aircraft from
the buoy, the refraction effects at the aircraft and at
the buoy will be essentially identical, allowing the technique to be used. In addition to providing the refraction data, the receiver will also provide the aircraft's
position; these data will be used to ensure accurate
dropping of the sonobuoys in the intended reentry
area.
The acoustic processer determines the acoustic transmission time between the various buoys in the array.
The information is used to compute the relative position of the buoys. The position data and the transdigitizer position data are fed to the control processor,
which combines the information to provide the geodetic position of the buoy. The acoustic processor also
processes the splash data from the reentry bodies and
computes the geodetic coordinates of the reentry body
splash points. The computed results are recorded on
a computer-compatible magnetic tape recorder. Displays of buoy status are provided for the operator.
There is also a command transmitter that is used to
control the trans digitizer buoys.
Impro ved Sonobuoy System for Missile Impact Location
POSTMISSION PROCESSOR
A postmission processing system will also be developed. The processing facility will use many of the same
hardware and software modules as the aircraft systems
and will be equipped with a large minicomputer for
performing the various acoustic and position computations. A larger computer will allow more sophisticated software to be used in the calculations, resulting
in improved accuracy.
STATUS OF THE PROGRAM
The program has been under way for over two years
and is in phase two of a three-phase development program. During phase one, two sonobuoys were designed
and fabricated, along with a rudimentary version of
the aircraft equipment. For the phase one test, only
GPS-related equipment was provided. During that
phase, it was felt that there was no need to test the
acoustic portion of the system because similar techniques have been used in the past.
Testing of the phase one system was performed at
the St. Croix, V. I., test range in February 1983. Although two buoys were available, only one was
deployed in the ocean at a time. However, a second
trans digitizer was operated at the land-based collection site to provide reference data. The recording subsystem was capable of recording signals simultaneously
North error (ft)
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Figure 6 - Results of the navigational test at St. Croix. During a system test there , a transdigitizer assembly located at
a surveyed point was used to collect signals from the GPS
satellites in view. The data were processed to determine the
location of the point. Shown is the difference between the
computed position and the position as determined by a local survey , for a number of data samples collected over a
3-day period. The cause of the bias , which can readily be seen ,
has yet to be determined but may be in the local survey.
Volume 5, Number 4, 1984
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Figure 7 - Relative position results. During the test at St.
Croix, a reference transdigitizer located at the tracking site
was operated concurrently with a buoy in the water. Tied to
the GPS transdigitizer buoy by a 10 foot cable was a reference buoy provided by the test range. Its position was determined by an acoustic system operated by the test range.
The relative positions of the two transdigitizers were determined by the GPS signals and by the range acoustic system
which was tied to the local survey. Plotted on the figure i~
the difference between the two differences. Part of the error
results from the cable between the reference buoy and the
GPS transdigitizer buoy.
339
E. E. Westerfield et al. - Impro ved Sonobuoy System jor Missile Impact Location
from both transdigitizers . Only a single-channel tracker was available at the field site; it provided for system checkout, but detailed processing had to be performed after the test.
The results of the phase one test were very satisfactory. Figure 6 is a bullseye pattern showing a number
of typical fixes. The difference between the position
determined by GPS and a survey is shown. The slight
bias in the data can readily be seen; the source of the
bias has yet to be determined. Figure 7 is a bullseye
plot showing the error in the translocated buoy positions. To generate the plot, the positions of the reference transdigitizer and the buoy were first determined
by tracking the recorded signals from three or more
GPS satellites as received by each transdigitizer. The
data were processed, and the error in the computed
position of the land-based antenna was subtracted
from the computed position of the floating transdigitizer buoy. These positions were compared with the
acoustic position data provided by the St. Croix range
through the use of pinger buoys attached to the trans-
340
digitizer buoys via a 10 foot tether. The resulting
differences are plotted in Fig. 7. A significant part of
the error shown is due to the fact that the range buoy
could be as much as 10 feet from the trans digitizer
buoy. Note, however, that there is essentially no bias
in these data.
The second phase of the program consists of developing an improved buoy that has full capability (including the reception and generation of acoustic
signals) and prototype aircraft equipment. Testing of
the complete system, including developmental buoys
and aircraft equipment, will be carried out at St. Croix.
During the third phase of the program, operational
hardware will be fabricated. APL will generate a full
set of specifications that the Air Force will use to procure commercial production quantities of trans digitizer
and pinger buoys. APL personnel will serve as technical advisors to the Air Force for that procurement.
APL personnel will also oversee the fabrication of six
sets of aircraft equipment and will generate the operational software.
Johns Hopkins APL Technical Digest